1. Field of the Invention
This invention relates to polymeric dispersant additives for lubricating oils which may also be useful as viscosity index improvers exhibiting improved viscosity stability in oil upon storage. More particularly, this invention relates to copolymers of ethylene with one or more C.sub.3 to C.sub.28 alpha-olefins, preferably propylene, which have been grafted with ethylenically unsaturated acid moieties, e.g., maleic anhydride, and reacted with a polyamine to form multi-functional copolymeric reaction products, and said reaction products are then reacted with a viscosity stabilizing effective amount of a C.sub.12 to about C.sub.18 hydrocarbyl substituted dicarboxylic acid anhydride to inhibit or retard viscosity increase over an extended period of time of oil solutions of these additives.
2. Description of the Prior Art
Viscosity index (V.I.) improving high molecular weight ethylene copolymers derivatized with acid moieties such as maleic anhydride and amines to form a V.I. improver-dispersant oil additives are well known in the art. Thus, for example, U.S. Pat. Nos. 3,316,177 and 3,326,804 teach the general concept of grafting an ethylene-propylene copolymer with maleic anhydride and then reacting with a polyalkylene polyamine such as polyethylene amine. Subsequently, U.S. Pat. No. 4,089,794 was directed to using an oil solution for free radical peroxide grafting the ethylene copolymer with maleic anhydride and then reacting with the polyamine. This concept has the advantage that by using oil the entire reaction would be carried out in an oil solution to form an oil concentrate, which is the commercial form in which such additives are sold.
It is often found that during storage of oil solutions, particularly oil concentrates, of these various grafted ethylene copolymers the viscosity of the oil solution is increased. The source of this increase appears to be, at least in part, the chain extension of the polymer. Several solutions to this problem are described in the prior art. One solution is to use the polyamines and then to react the remaining unreacted primary amino groups with acid anhydrides of either C.sub.1 -C.sub.30 monocarboxylic acid, preferably acetic anhydride, or unsubstituted or C.sub.1 to C.sub.8 hydrocarbyl substituted dicarboxylic acid anhydrides of U.S. Pat. No. 4,137,185; or the sulfonic acids of U.S. Pat. No. 4,144,181. These materials act as polyamine group end-capping or inactivating agents to inhibit or limit chain extension. That is to say, the reaction product of the graft copolymer of the ethylenically unsaturated dicarboxylic acid material and polyamine is post-treated with these acid materials to inhibit chain extension and viscosity increase of the imide grafted ethylene copolymer.
While the end-capped or chain extension terminated grafted ethylene copolymers disclosed in U.S. Pat. Nos. 4,137,185 and 4,144,181 are generally quite useful, they possess some problems under certain conditions. Thus, for example, reacting a monocarboxylic acid anhydride, e.g., acetic anhydride, with the polyamine yields an acid by-product, e.g., acetic acid. This acid is deleterious to engine operation, causing corrosion, and generally needs to be removed from the oil solution of the V.I. improver-dispersant.
Furthermore, the use of the unsubstituted or lower hydrocarbyl substituted dicarboxylic acid anhydrides of U.S. Pat. No. 4,137,185 generally does not remedy, and may sometimes even contribute to, the formation of haze in oil solutions of the imide grafted ethylene copolymer. This is due to the fact that when using free radical initiators with mineral oil as the grafting medium a proportion of the oil molecules in turn become grafted with the ethylenically unsaturated moiety, e.g., maleic anhydride, and upon subsequent reaction with the amine these grafted oil particles tend to become insoluble and to form haze. Upon the addition of the hydrocarbyl substituted dicarboxylic acid anhydrides to this oil solution to inactivate the unreacted primary amine groups of the imide grafted ethylene copolymer, a certain proportion of the anhydride reacts with the free amine groups of the grafted oil particles. Since the prior are anhydrides are either unsubstituted or lower hydrocarbyl substituted they do not facilitate the solubilization of these insoluble grafted oil particles. However, since the dicarboxylic acid anhydrides of the instant invention are substituted with a C.sub.12 to about C.sub.18 hydrocarbyl group they tend to facilitate the solubilization of the insoluble grafted oil particles, thereby decreasing haze.
The use of the sulfonic acids of U.S. Pat. No. 4,144,181 may also result in the formation of undesirable acid by-products, e.g., hydrogen sulfide.
It has been found that improved viscosity stability upon storage of oil solutions, particularly oil concentrates, of the imide grafted ethylene copolymers can be achieved while remedying the problems of acid by-product formation and hazing accompanying the utilization of these prior art polyamine group end-capping or chain extension terminating agents by utilizing a C.sub.12 to C.sub.18 hydrocarbyl substituted dicarboxylic acid anhydride as the polyamine end-capping or chain extension terminating agent.
The prior art, i.e., U.S. Pat. Nos. 4,517,104 and 4,632,769, also teaches the use of certain carboxylic alkenyls during the imidation of the ethylenically unsaturated carboxylic acid grafted ethylene copolymer. However, these carboxylic alkenyls are used for a different purpose, serve a different function, and result in a different end product than that of the present invention. These patents disclose grafting ethylene copolymers with ethylenically unsaturated carboxylic acid moieties, preferably maleic anhydride moieties, and then reacting the grafted copolymer with a mixture containing polyamines having two or more primary amine groups and a carboxylic alkenyl component, preferably alkylene polyamine and alkenyl succinic anhydride; or the grafted ethylene copolymer can be reacted with the already formed salts, amides, imides, etc. of said polyamine and carboxylic alkenyl component, preferably imides of alkylene polyamine and alkenyl succinic anhydride. Thus, the carboxylic alkenyl component is one of the reactants and enters into in the imidation reaction involving the polyamine and the ethylenically unsaturated acid grafted ethylene copolymer.
U.S. Pat. No. 4,517,104 discloses that the carboxylic alkenyl component can be, inter alia, a C.sub.12 to C.sub.49, preferably C.sub.16 to C.sub.49 hydrocarbyl succinic anhydride or acid. U.S. Pat. No. 4,632,769 discloses that the carboxylic alkenyl component can be, inter alia, a C.sub.22 to C.sub.28 hydrocarbyl substituted succinic anhydride. However, these patents teach that the carboxylic alkenyl component and the polyamine are reacted simultaneously with the grafted ethylene copolymer or that the grafted ethylene copolymer is reacted with the prefomed salts, amides, imides, etc. of said polyamines and carboxylic alkenyl component.
This is in sharp contrast to the practice of the instant invention wherein the preformed imide grafted ethylene copolymer is reacted with the C.sub.12 to about C.sub.18 hydrocarbyl substituted dicarboxylic acid anhydride. The main purpose of reacting either the preformed salts, amides, imides, etc. of the polyamine and the carboxylic alkenyl component or a mixture of the polyamine and carboxylic alkenyl component with the ethylenically unsaturated acid grafted ethylene copolymer is to have the carboxylic alkenyl moiety enter into the imidation and/or amidation reaction thereby preventing excessive cross-linking via the unreacted primary amine groups of the polyamine thereby controlling the molecular weight of the grafted ethylene copolymer product. Generally, the greater the amount of the carboxylic alkenyl component present during the imidation and/or amidation reaction of the ethylenically unsaturated acid grafted ethylene copolymer and the polyamine the lower the degree of cross-linking and the lower the molecular weight of the final grafted ethylene copolymer. The smaller the amount of said carboxylic alkenyl component present during the imidation and/or amidation reaction, the greater the degree of cross-linking and the higher the molecular weight of the grafted ethylene copolymer product.
While not wishing to be bound by any theory this is believed to be due to stoichiometric considerations of the imidation and/or amidation process used to form these grafted ethylene copolymers. There is generally a competing reaction between the polyamines and the acid moiety of the ethylenically unsaturated acid grafted ethylene copolymer on the one hand and the carboxylic alkenyl component on the other hand. Using less of the carboxylic alkenyl component results in more of the polyamine being available to react, via the unreacted primary amine groups, with the acid moiety of the acid grafted ethylene copolymer. This results in an increased degree of cross-linking and an increased molecular weight. If, on the other hand, more of the carboxylic alkenyl component is utilized the amount of the polyamine containing at least one unreacted primary amino group, available to react with the acid moiety of the acid grafted ethylene copolymer to form the imide grafted ethylene copolymer, is reduced thereby resulting in a lower degree of cross-linking and a lower molecular weight. Therefore, since the carboxylic alkenyl component functions as a cross-linking and molecular weight regulator and not as a viscosity stabilizing agent the viscosity of oil solutions, particularly oil concentrates, of these grafted ethylene copolymers also tends to increase with time.